In practical fuel cell systems, the output of a single fuel cell is typically less than one volt, so connecting multiple cells in series is required to achieve useful operating voltages. Typically, a plurality of fuel cell stages, each stage comprising a single fuel cell unit, are mechanically stacked up in a “stack” and are electrically connected in series electric flow from the anode of one cell to the cathode of an adjacent cell via intermediate stack elements known in the art as interconnects and separator plates.
A solid oxide fuel cell (SOFC) comprises a cathode layer, an electrolyte layer formed of a solid oxide bonded to the cathode layer, and an anode layer bonded to the electrolyte layer on a side opposite from the cathode layer. In use of the cell, air is passed over the surface of the cathode layer, and oxygen from the air migrates through the electrolyte layer and reacts in the anode with hydrogen being passed over the anode surface, forming water and thereby creating an electrical potential between the anode and the cathode of about 1 volt. Typically, each individual fuel cell is mounted, for handling, protection, and assembly into a stack, within a metal frame referred to in the art as a “picture frame”, to form a “cell-picture frame assembly”.
To facilitate formation of a prior art stack of fuel cell stages wherein the voltage formed is a function of the number of fuel cells in the stack, connected in series, a known intermediate process for forming an individual fuel cell stage joins together a cell-picture frame assembly with an anode interconnect and a metal separator plate to form an intermediate structure known in the art as a fuel cell cassette (“cassette”). The thin sheet metal separator plate is stamped and formed to provide, when joined to the mating cell frame and anode spacers, a flow space for the anode gas. Typically, the separator plate is formed of ferritic stainless steel for low cost. In forming the stack, the cell-picture frame assembly of each cassette is sealed to the perimeter of the metal separator plate of the adjacent cassette to form a cathode air flow space and to seal the feed and exhaust passages for air and hydrogen against cross-leaking or leaking to the outside of the stack.
The separator plate provides for fluid flow separation between the anode and cathode of adjacent cells in the fuel cell stack, and also provides part of an electrically conductive path connecting the anode from one cell in series with the cathode of an adjacent cell. In some fuel cell stack designs, the separator plate itself is configured on one or both sides to provide a three-dimensional structure that provides contact with the electrode of an adjacent fuel cell at a number of locations so that electrical connectivity, with spaces between the points of contact so that fluid (air or fuel) can flow along the surface of the electrode. In other designs, a separate interconnect structure is disposed in the stack between separator plate and the adjacent fuel cell(s).
To optimize the robustness of the electrical contact to the anode or cathode of a fuel cell, an interconnect should contact the cell surfaces at many locations as possible to minimize electrical resistance, and the points should be close together to minimize current path length and variations in current density that can lead to parasitic electrical losses. In addition, and largely in conflict with the requirement for numerous closely-spaced points of electrical contact, the interconnect should provide a flow path for air or fuel with as much exposure to the cell surfaces as possible for efficient cell operation. It is also desirable to minimize flow restrictions to reduce pumping losses in the fuel cell system that can lower power output and operating efficiency, while at the same time minimizing the interconnect height to provide compactness to the fuel cell. It is also desirable to provide the interconnect with a robust structure that is stiff in the direction perpendicular to the cell surface in order to minimize dimensional changes (creep) of the fuel cell stack structure due to the high operating temperature, as well as provide for efficient and reproducible manufacturing processes at low cost.
Various types of structures have been disclosed as fuel cell interconnects. Woven wire meshes have been proposed, but are relatively expensive, have high flow restriction and point contact to the cell surfaces, making effective electrical contact difficult and have poor dimensional stability. Sheet metal with formed dimples or ridges have also been disclosed. Although such structures can be formed inexpensively into the separator sheet itself, often eliminating the need for one or both of the separate interconnect structures, they have poor physical integrity, with low strength in the direction perpendicular to the cell surface, resulting in creep that leads to loss of electrical contact over time. Thick sheet metal plates with machined features have also been proposed, but the spacing of the electrical contacts is often greater than desired or covers too much surface area due to limitations of machining. These effects can be minimized with more finely detailed machining, but this can drive the cost up considerably and also can compromise tool strength. Alternatively these thick structures can be formed without machining from multiple essentially 2-dimensional pieces that are stacked and brazed together, but this also drives up manufacturing costs considerably.
Accordingly, it would be desirable to provide new alternatives for fuel cell interconnects.